1. Field of the Invention
The present invention relates to methods of removing and replacing catalyst in a multi-reactor series. More particularly, the invention relates to such methods in which a predetermined quantity of at least partially spent catalyst is removed from the first of a plurality of reaction vessels and is introduced into a second reaction vessel, and a like amount of fresh catalyst is introduced into the first reaction vessel.
2. Description of Related Art
The use of solid catalysts in industrial catalytic reactors is well known, especially in the field of petroleum refining. Various catalytic distillation processes, fluid catalytic cracking processes, catalytic hydrocracking processes, catalytic reforming processes and catalytic hydrotreating processes, for example, are well documented in the literature. The catalysts employed in those processes have a variety of chemical compositions and take different forms, including powders or particles in fixed, moving, fluidized, or slurry beds, and porous or permeable monoliths, foams, gauzes, and the like. Depending on the requirements of a particular process, multiple catalyst beds may be employed in a single catalytic reactor, or multiple reactors may be employed in parallel and/or serial configurations, with respect to the path of the reactant and product fluids and/or gases. Regardless of the reactor configuration, over a period of time in use on stream, a catalyst typically loses part or all of its activity for catalyzing the desired chemical reaction(s). In some cases, this catalyst inactivation is due to gradual poisoning by a contaminant (e.g., sulfur, or metals) in a reactant feedstock, or by deposition of the product of an undesired side reaction (e.g., “coking”). When the efficiency or quality of the reactor output diminishes sufficiently, it becomes necessary to remove the spent catalyst and replace it with fresh catalyst in order to restore the desired level of productivity of the reactor.
A variety of catalyst removal and replacement strategies are employed conventionally. One of the simplest ways consists of halting the process after the efficiency of the reactor has deteriorated to an unacceptable level, and, if appropriate, regenerating in situ the catalyst that is retained inside the reactor column. Alternatively, the catalyst is removed and replaced. One drawback of these approaches is that the entire reactor operation must be halted during catalyst replenishment. Regeneration of the catalyst can take a considerable amount of time, causing appreciable reactor downtime and loss of productivity. Moreover, the catalyst that is removed is often only partially spent, and is replaced by fresh catalyst or routed to a regeneration system, which increases the cost of the process.
Many ways of unloading and reloading particulate catalysts without halting the progress of the process have been described in the literature. For example, in U.S. Pat. No. 5,198,196 a catalytic distillation process is described in which catalyst particles are drawn off into a separating device, regenerated in a separate system, and then returned to the reaction vessel as a slurry of fresh catalyst.
U.S. Pat. No. 3,730,880 describes a reaction vessel in which the reactants flow either co-current or counter-current to the catalyst. Fresh catalyst moves from the upper fresh catalyst introduction vessel to the upper region of the reaction vessel by periodically closing valves between the vessels. Likewise, spent catalyst is removed from the lower region of the reaction vessel into a lower catalyst disposal vessel where the spent catalyst is no longer involved in the reaction in the reaction vessel. In a counterflow system, this permits removal of catalyst from the lowermost or first stage vessel, where the raw feed stock originally contacts the catalyst.
U.S. Pat. No. 3,725,248 describes a series flow reactor system in which all reactors remain on stream during a reforming process. Each reactor contains an annular dense-phase moving bed of catalyst particles and a catalyst collector. The catalyst is processed through the reactor system counter-current to the direction of reactant flow. Fresh or regenerated catalyst is charged to the top of the reactor. A like quantity of partially spent catalyst is withdrawn from the bottom of each reactor and is added to the adjacent upstream reactor after regeneration. Thus, unlike the present invention, in which the age distribution of the particles within the reactor is substantially uniform, the age distribution in this plug-flow catalyst system is very old spent catalyst on the bottom with fresh catalyst being fed into the top. Because the catalyst is fed through the reactor linearly, in a plug-flow arrangement, it is necessary to regenerate the catalyst before introducing it to the subsequent reactors.
U.S. Pat. No. 5,733,440 describes a method for on-stream catalyst replacement during hydroprocessing of a hydrocarbon feed stream. Hydrogen gas and hydrocarbon liquid are introduced at a rate insufficient to levitate or ebulliate the catalyst bed. The substantially packed catalyst bed continually flows in a plug-like manner downwardly through the reactor vessel. Fresh catalyst is introduced at the top of the catalyst bed by laminarly flowing the catalyst in a liquid stream on a periodic or semicontinuous basis. Catalyst is similarly removed by laminarly flowing catalyst particles in a liquid stream out of the bottom of the catalyst bed. The rate at which catalyst is removed from the reaction zone, and the rate of catalyst replacement to the reaction zone, is established by a number of economic and operating factors, which include maintaining a desired average level of catalytic upgrading activity. Withdrawal of about 10–25 weight percent of the catalyst at a time from the bottom of a non-ebulliated catalyst bed, and replacement with fresh catalyst at the top of the bed, is described.
Many catalytic reactors in use today utilize ebulliated, slurry, or expanded catalyst bed reactor technology. In a hydroprocessing unit, for example, a hydrocarbon feed stream and hydrogen gas flow upwardly through a dilute phase reaction zone of catalyst in random motion. The catalyst may be replaced by continuous or periodic, onstream removal of catalyst from the reaction vessel followed by addition. Such counterflow systems have also been used because of the relative ease of withdrawing limited amounts of the ebulliated catalyst in a portion of the reacting hydrocarbon and hydrogen fluids, particularly where such turbulent flow of the catalyst is needed to assist gravity drainage through a funnel-shaped opening into a central pipe at the bottom of a vessel. According to U.S. Pat. No. 5,472,928 catalyst replacement rates for ebulliated bed reactors are based on maintaining catalyst equilibrium conditions necessary to maintain processing objectives. Difficulties with respect to withdrawal of expended catalyst occur due to commingling of fresh or partially expended catalyst with expended catalyst withdrawn from the bottom of the catalyst bed in ebulliated bed systems.
U.S. Pat. No. 4,902,407 describes a method of controlling catalyst inventory in an ebulliated bed process for treating hydrocarbon liquids with hydrogen. Pressure differentials are measured to calculate a catalyst inventory characterization factor. Aged catalyst is withdrawn and fresh catalyst added in an amount to reestablish the value of the factor and to maintain the desired catalyst to oil ratio.
U.S. Pat. No. 3,470,090 describes a method of operating a non-regenerative fixed bed reforming process. A multiple fixed-bed reactor system is illustrated in which the catalyst moves down through the reactor in a plug-flow system. The spent catalyst is removed from the bottom and fresh catalyst is added to the top simultaneously to each of the reactors, either continuously or periodically, using a lock hopper system or a screw conveyor system. U.S. Pat. No. 4,167,474 suggests modifying that method to the extent that the catalyst particles withdrawn from a given reaction zone are transported to the next succeeding reaction zone, while the catalyst withdrawn from the last reaction zone may be transported to a suitable regeneration facility, as illustrated in U.S. Pat. Nos. 3,839,197 and 3,839,196.
U.S. Pat. No. 5,589,057 describes a method for extending the life of a hydroprocessing catalyst. The catalyst bed continually flows in a plug-like manner downwardly through a single reactor vessel, and catalyst is removed on a periodic or semicontinuous basis by laminarly flowing catalyst particles in a liquid stream out of the bottom of the catalyst bed. The high-activity less dense fraction of catalytic particulates are separated out of the removed catalyst and are subsequently mixed with fresh catalyst and returned to the reactor vessel.
U.S. Pat. No. 5,925,238 describes a catalytic multi-stage hydrodesulfurization process with cascading rejuvenated catalyst. Used catalyst having a certain catalyst equilibrium age is withdrawn from the second stage reactor, rejuvenated, and then cascaded forward and added to the first stage reactor. Sufficient fresh make-up catalyst is added to the second stage reactor to replace the used catalyst withdrawn there, and only sufficient fresh catalyst is added to the first stage reactor to replace any catalyst transfer losses.
A drawback of most present day catalyst replacement regimes is that they fail to adequately conserve the amount of new or fresh catalyst that is consumed in the overall operation. This is especially problematic in slurry bubble or fluidized bed reactors, which typically employ higher reactant flow rates and space velocities, and use smaller, more highly active catalyst beds than their fixed bed or plug-flow counterpart reactors. While the fouled or spent catalyst can usually be expected to concentrate in the fixed bed region that first contacts the reactant fluids or gases, and can be selectively removed by a counterflow catalyst removal system, this is not possible with slurry bubble fluidized catalyst beds. Typically, in a slurry bubble fluidized bed reactor, when spent catalyst is frequently or continuously removed from a process and replaced with new catalyst, in order to maintain high efficiency of operation, the withdrawn catalyst ranges in age from brand new to very old catalyst. In existing slurry bubble or fluidized bed catalytic processes, oftentimes more fresh catalyst is added to the catalytic reactor than is really necessary in order to maintain an acceptable level of process performance. What is needed is a way to optimize the catalyst withdrawal and replacement program in a serial multi-reactor catalytic process to reduce the amount of expensive catalyst consumed over the run period while maintaining an acceptable catalyst age profile. It is also desirable in some instances to minimize the catalyst age profile while also reducing the amount of catalyst used.